1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copying machine, a printer, or a facsimile machine, which can perform image formation according to an electrophotographic method.
2. Description of the Related Art
There is a conventional image forming apparatus, such as a copying machine or a page printer, which can perform image forming processing according to an electrophotographic method. The image forming apparatus according to the electrophotographic method can form a toner image on a material (e.g., paper) using electrostatic force. The image forming apparatus includes a fixing device that can apply heat and pressure to the toner image formed on the material to discharge a fixed toner image as an output image.
An electrophotographic process color image forming apparatus is widely used as one of the electrophotographic image forming apparatuses, which includes a transfer belt that can realize advanced functions such as color image formation and speedy printing.
FIG. 9 illustrates a schematic configuration of a conventional image forming apparatus including an intermediate transfer member in the form of an intermediate transfer belt 31. The image forming apparatus illustrated in FIG. 9 is an example of the color image forming apparatus that performs electrophotographic processes. The intermediate transfer belt 31 is stretched by three tension rollers 8, 10, and 34. The image forming apparatus illustrated in FIG. 9 includes yellow (Y), cyan (C), magenta (M), and black (Bk) process cartridges 32a, 32b, 32c, and 32d, which are independently disposed.
Each process cartridge includes a drum-shaped image carrier (hereinafter, referred to as a photosensitive drum) 2a to 2d. The photosensitive drum 2a to 2d is driven to rotate in a direction indicated by an arrow at a predetermined circumferential speed (i.e., a process speed). The photosensitive drum 2a to 2d is subjected to charging processing in the process of rotation described above. A primary charging device 3a to 3d can uniformly charge the photosensitive drum 2a to 2d to have an electric potential of a predetermined polarity.
Next, an image exposure unit 4a to 4d performs image exposure processing to form an electrostatic latent image corresponding to a first color component image (e.g., yellow component image), which is a target color image, on the photosensitive drum 2a to 2d. Next, a developing device 5a to 5d (more specifically, a yellow developing device 5a) develops the electrostatic latent image at a developing position to visualize the developed image as a toner image on the photosensitive drum 2a. The yellow toner image formed on the photosensitive drum 2a is then transferred from the photosensitive drum 2a to the intermediate transfer belt 31 (i.e., an elastic member having a medium resistance) at a primary transfer portion.
A primary transfer member 14a to 14d disposed on an inner circumferential surface side of the intermediate transfer belt 31 in a confronting relationship with the photosensitive drum 2a to 2d, and the intermediate transfer belt 31 cooperatively constitute the primary transfer portion.
Similarly, the cyan, magenta, and black process cartridges perform the above-described process for transferring the toner image formed on the photosensitive drum 2a to 2d to the intermediate transfer belt 31. As a result, a full color (i.e., 4-color) toner image is formed on the intermediate transfer belt 31. A secondary transfer member 35 integrally transfers the full color (i.e., 4-color) image formed on the intermediate transfer belt 31 to a transfer material S at a secondary transfer portion. A fixing apparatus 18 fuses and fixes the transferred image to form a color print image.
A cleaner apparatus removes secondary transfer residual toner (i.e., any toner remains on the intermediate transfer belt 31 without being transferred to the transfer material S) off the intermediate transfer belt 31 at the secondary transfer portion. The cleaner apparatus includes an elastic belt cleaning blade 33 disposed to face in a counter fashion against a rotational direction of the intermediate transfer belt 31. After the above-described image formation processing is completed, the primary transfer member 14a to 14d is separated from the intermediate transfer belt 31 and the intermediate transfer belt 31 stops rotating.
In general, an image forming apparatus capable of forming a color image can select its operational mode between a multicolor mode and a mono-color mode. The image forming apparatus selects the multicolor mode to perform image formation using a plurality of image formation units. The image forming apparatus selects the mono-color mode to perform image formation using only one image formation unit.
According to the image forming apparatus capable of switching its operational mode between the multicolor mode and the mono-color mode, if the photosensitive drum of an image formation unit that does not function for image formation in the mono-color mode is continuously engaged with the transfer belt, a surface of the photosensitive drum may be abraded by the transfer belt.
Hence, as discussed in Japanese Patent Application Laid-Open No. 10-207151, to prevent the photosensitive drum from being abraded when it does not function in the mono-color mode, it is useful to provide a mechanism for separating the photosensitive drum from a rotating transfer belt when the photosensitive drum does not function in the mono-color mode. According to the configuration discussed in Japanese Patent Application Laid-Open No. 10-207151, the photosensitive drum can be separated from the transfer belt by separating a transfer member disposed in an opposed relationship with the photosensitive drum that does not function in the mono-color mode.
However, in a case where the primary transfer member 14a to 14d is configured to be driven by the transfer belt 31, a surface of the primary transfer member may be frictionally abraded while the primary transfer member is frictionally engaged with the transfer belt when the primary transfer member is separated in a switching operation from the multicolor mode to the mono-color mode.